Conventionally, when obtaining medical images by scintigraphy, a tracer substance containing a radioactive isotope is injected into the patient and attaches preferentially to the organ that is to be observed. The gamma rays emitted by the radioactive isotope are picked up by an image acquisition system of the gamma camera type.
For conventional image acquisition, this image-forming system includes a collimator which captures the radiation emitted in a given direction and the collimator is followed by a scintillator associated with a detector assembly.
In order to acquire photographic type images, the collimator of the gamma camera is replaced by a pin-hole cone, i.e. a cone which is closed by a tungsten end fitting having a hole through the center thereof which passes only a portion of the gamma radiation emitted by the zone under observation, with the remainder of the cone providing isolation from the gamma radiation emitted by the remainder of the body. Point images of radioactive isotope concentration in the zone under observation are thus formed by the "objective lens" constituted by the hole, and the scintillator placed at the base of the cone then forms a magnified image on the detector assembly. Resolution is thus improved over conventional acquisition and the smaller the hole the greater the accuracy.
In general, a given apparatus may be fitted either with a conventional collimator forming a 1 to 1 image of the zone under observation or else with a pin-hole cone. It is difficult to optimize the characteristics of the cone. Depending on the injected substance, the energy of the emitted gamma radiation varies over a wide range. In order to stop "low" energy radiation, a cone made of 7 mm thick lead suffices, whereas for stopping "high" energy radiation, it is necessary for the thickness of the cone to be as much as 20 mm. Several different pin-hole cones may therefore be provided for the same apparatus depending on the energy of the emitted radiation, which in turn depends on the substance injected.
These interchangeable components are heavy and machines are required to move them. A conventional collimator or a pin-hole cone commonly weighs about 100 kg (where the total weight of a gamma camera is about 1.5 metric tons). Even when a low energy cone is used, account must be taken of the fact that the camera needs to be balanced, so counterweights are provided. As a general rule, such counterweights are installed permanently. Consequently, the interchangeable parts, i.e. the collimator and the various cones, must all have approximately the same weight. The cones are therefore provided with bases that are designed to obtain the necessary weight.
An object of the invention is to provide a photographic type acquisition gamma camera in which the pin-hole cone is adaptable to the receive radiation energy while being much easier to install than the equivalent cone in conventional gamma cameras.